Archer and Flynn, U.S. Pat. No. 3,361,700, disclose that a family of alkoxyalkyl esters of diiodobenzoic acid are radiopaque and suitable to plasticize vinyl resins into a form useful to manufacture tubings for catheters and similar products. Flynn, U.S. Pat. No. 3,645,955 discloses that di- and tetraiodoesters used alone or in combination with the alkoxyalkyl diiodoesters are superior for this purpose because they show less tendency to exude and lower concentrations provide a better balance between flexibility and stiffness. In the copending application of Flynn, Ser. No. 862,773, there are described certain multi-wall tubing constructions, with a passing mention of polyurethane as a material of construction, but no suggestion is made that this be combined with any radiopacifier, especially the iodoesters mentioned above. Burlis et al, U.S. Pat. No. 3,752,617 disclose methods for making multiwall tubing, co-tapered along its length, but use plastics other than polyurethane, and make no mention of any additives, specifically by name, to produce different X-ray sensitive characteristics. The foregoing patents and application are all incorporated herein by reference.
While the iodoester opacified vinyl resin compositions are quite suitable for the production of tubing of simple types useful for intubation sets and catheter needles, they are not completely satisfactory for production of shaped devices. For example, if flared, or if formed into curved tips, the shapes tend to revert to straight tubing--a so-called loss of plastic memory effect. It has now been found that if the vinyl resin is replaced partially or completely by a thermoplastic polyurethane, the iodoester radiopacified compositions are amenable to the induction of complex shapes--and they'll stay that way. Such tubings provide catheters eminently suitable for selective renal arteriography and for celial arteriography, and also for the manufacture of pig-tail catheters. Surprisingly, the iodoesters do not show the same tendency to overplasticize the urethane esters as they do the vinyl esters, and, moreover, there is lesser need to use a radioparent plasticizer with urethanes and, in fact, it is preferred to omit a radioparent plasticizer completely. As will be shown, the new compositions lend themselves well to the formation of highly advantageous multi-wall tubing constructions. In one feature of the invention, a thin, inner core of iodoester-radiopacified polyurethane and a thicker outer core of plasticized vinyl resin eliminates extraction problems with fluids passing through the core. In another feature, an inner core of nylon or polypropylene will provide an outer jacket of iodoester radiopacified urethane or urethane-vinyl with stiffness, but much less tendency to kink in a multiwall construction; and such tubes are also easily manufactured in a co-tapered wall thickness construction in which the variations in twisting resistance provide enhanced ease of insertion through torturous body passages.
In summary, optically clear radiopaque formulations based on thermoplastic polyurethane (TPU) according to this invention have the following distinct advantages over vinyl (PVC)-based compositions of the prior art:
(i) ease and flexibility in formulation and processing: In contrast to PVC formulations, TPU does not require a primary plasticizer or heat stabilizer. PVC-based systems, in general, are temperature and shear sensitive, and consequently, the melt processing range is rather limited. PA1 (ii) potentially less toxic: PVC is more susceptible to thermal/shear induced degradation. In plasticized PVC formulations, both the plasticizer and the residual monomer are subject to migration and extraction phenomena. (The newer grades of PVC, admittedly, contain insignificant quantity of residual monomer). In recent years, PVC has come under increased public scrutiny and criticism. PA1 (iii) physical and functional properties: TPU can be formulated to obtain a much broader range of physical properties, if desired. Distinct advantages of TPU formulations will be in higher tensile strength, elongation and excellent abrasion resistance and tear strength, and especially in retention of plastic memory. PA1 (iv) better blood compatibility: In many tests, the thrombo-resistance of TPU has been shown to be superior to PVC. PA1 (v) better ability to accept heparinization.